The operational life of bearings can be influenced by the method and efficiency of lubrication delivery to the bearing. An oversupply of lubrication can lead to increased churning and heat generation which may lead to premature degradation of the lubricant and failure of the bearing. Conversely, an under supply of lubrication leading to “starved lubrication conditions” can result in increased contact friction, leading to damage and premature bearing failure.
Oil is a typical lubricant used to lubricate bearings. Conventional oil delivery methods to a bearing include greasing, oil mist, oil bath, and oil jet arrangements. The choice of particular method is usually dictated by the operational environment and conditions of the bearing.
Bearings in a typical gas turbine engine drive shaft arrangement have bearings stacked in a vertical arrangement, often relying on the cascading of oil from the upper to the lower part of the system. However, as demands on power transmission and heat dissipation ever increase, more efficient methods of entraining oil to specific locations are required.
FIG. 2 shows an example of a prior art roller bearing 30, with an under race oil feed 32 and an inner piloted cage 34, provided between an outer body 28 and an inner body 29. FIG. 2 provides a cross-sectional view of e.g. an upper portion (only) of a roller bearing assembly, viewed in a vertical plane containing the rotational axis of the cage 34.
The cage 34 provides a pocket for retaining a rolling element 36, such as a roller as in this example or a sphere (e.g. a ball bearing). Rolling element 36 engages with inner raceway 38, provided by inner body 29, and engages with outer raceway 40, provided by outer body 28. A series of pockets are provided by the cage, around the circumference of the cage.
The cage 34 includes a cage inner surface 35, acting as a pilot surface, for cooperating with the outer surface 29a, of inner body 29, acting as a cooperating pilot surface. The respective pilot surfaces 35 and 29a cooperate to maintain the cage 34 in coaxial arrangement with the inner body 29 (and thus also the outer body 28 by virtue of rolling bearing elements 36). There is typically a significant clearance between outer surface 35′ of cage 34 and inner surface 28a of the outer body 28. In other words, in a typical inner piloted cage, surfaces 35′ and 28a do not cooperate.
To prevent failure of the bearing 30, the rolling element 36 and the pilot surfaces 35 and 29a require suitable lubrication, as discussed above.
In the particular example shown in FIG. 2, the under race oil feed 32 is typically directed to the roiling element 36 of the bearing by the cage (particularly, via the pilot interface generated between the cooperating pilot surfaces 35 and 29a) and the action of gravity. Additionally, or alternatively, cage wings can be provided, which extend from the cage in the axial direction, and which form weir members to promote a feed of oil towards the rolling elements instead of away from them.
However, the architecture surrounding the bearing is not always suitable to permit the use of an inner piloted cage due to the space constraints and assembly considerations. This may be especially true in a gas turbine engine, where the surrounding architecture is particularly complex.
So, in alternative arrangements, an outer piloted cage may be provided. An outer piloted cage is dissimilar to the inner piloted cage shown in FIG. 2 in that a significant clearance is typically provided between the inner surface 35 of the cage 34 and the outer surface 29a of inner body 29. Instead, outer surface 35′ of cage 34 provides the pilot surface of the cage, and inner surface 28a of the outer body 28 provides the cooperating pilot surface, whereby cooperation between the pilot surfaces 28a and 35′ maintain the cage in coaxial arrangement with the outer body 28 (and thus the inner body 29 by virtue of rolling bearing elements 36).
To prevent failure of the bearing 30, the rolling element 36 and the pilot surfaces 35′ and 28a require suitable lubrication. However, an outer piloted cage bearing assembly increases the clearance (distance) between the lubricant (oil) feed supplied by the inner body 29 (the under race oil feed 32) and the cage inner surface 35.
Thus, contrary to the inner piloted cage arrangement, which has a tight clearance between the cage and the inner body to control directly the flow of oil (lubricant) to the working contacts of the bearing 30, the increased clearance associated with an outer piloted cage arrangement leads to a greater dispersal of oil (lubricant) before and after it comes in to contact with the cage.
These factors reduce the overall lubricant “catch efficiency” of the cage. In other words, they can led to a reduction in the effective amount of oil (lubricant) which is directed towards the working contacts of the bearing 30 (such as the rolling elements and/or the pilot surfaces) when compared with an inner piloted cage arrangement. As discussed above, this can lead to catastrophic failure of the bearing.
Additionally, increasing level of oil in the system to compensate for the relative reduction in oil (lubricant) delivery to the bearing 30, can compromise the oil (lubricant) delivery to other features in the arrangement and may reduce the performance and reliability of the overall arrangement.